In a liquid crystal display device, a liquid crystal alignment film has a role to align liquid crystal in a certain direction. At present, a principal liquid crystal alignment film industrially utilized is prepared by applying a polyimide type liquid crystal aligning agent comprising a solution of a polyamide acid (also called a polyamic acid) as a polyimide precursor or a polyimide obtained by imidizing the polyamide acid, to a substrate to form a film. Further, in the case of parallel alignment or tilt alignment of the liquid crystal relative to the substrate surface, a surface orientation treatment by rubbing is further carried out after film formation.
In order to improve the display properties of a liquid crystal display device, the structure of the polyamic acid or the polyimide is changed variously and optimized, resins differing in the properties are blended, or additives are added, to improve the liquid crystal alignment property, to control the pretilt angle, or to improve electric properties, etc., and for further improvement in the display properties, various techniques have been proposed. For example, Patent Document 1 proposes to employ a polyimide resin having a specific repeating structure in order to obtain a high voltage retention. Further, Patent Document 2 proposes to shorten the time till the residual image disappears, by using a soluble polyimide having a nitrogen atom in addition to the imide group, against the image retention phenomenon.
Further, the liquid crystal alignment film also has a role to impart a certain angle of inclination (pretilt angle) to the liquid crystal, and to impart the pretilt angle, a diamine containing side chains, etc., have been proposed, and impartment of the pretilt angle becomes an important object in development of a liquid crystal alignment film (Patent Documents 3 to 6).
Further, a liquid crystal alignment film has been desired such that the film is less likely to be peeled or scraped, when rubbing treatment is carried out as a process to impart uniaxial alignment property of liquid crystal molecules to a polyimide film (Patent Documents 7 to 9).
In recent years, along with the progress in high performance of a liquid crystal display device, increase in the area, energy saving of a display device, etc., a liquid crystal display device is used in various environments, and properties required for a liquid crystal alignment film are also diversified in a high level. Particularly, along with an increase in the size of a display, light and heat of the backlight tend to be intense, and development of a material resistant to deterioration by light or heat has been desired.